Pre-Analytic Issues in Laboratory Medicine

Transcription

1 Pre-Analytic Issues in Laboratory Medicine Daniel J. Fink, MD, MPH Director, Core Laboratory New York Presbyterian hospital Columbia University Medical Center October 3, 2006 Learning Objectives Pre-Analytic Issues in Laboratory Medicine What is the impact of posture on laboratory analytes? Be able to describe the common additives to blood specimens and when they are used Be able to describe the common causes of hemolysis and clot formation in the preanalytic setting Pre-Analytic or Extra-Terrestrial? Phases of Testing Pre-Analytical Test Ordering, Specimen Collection, Specimen Handling Analytical Test Performance, Quality Control, Result Review Post-Analytical Result handling, Result Communication, Result Interpretation Why are Pre-Analytical Issues Important? Abnormal test results usually are attributed to disease. This is not always the case. Important considerations in interpreting laboratory results are preanalytical handling (e.g. container, collection method, mixing) and biological variation. Analytical variability can also affect interpretation of a result Why are Pre-Analytical Issues Important? Biological Variation is due to factors inherent to the patient that may or may not be controllable Preanalytical variation is due to factors external to the patient affecting laboratory specimens prior to testing. Being aware of these factors and following proper procedures will minimize these effects, allowing for more reproducible, accurate results and interpretable results. 1

2 Biologic Variability Test Ordering Age, sex, race Alkaline phosphatase in children Neutrophil counts in neonates Creatinine, CK in females, blacks Diurnal variation Glucose values obtained during an oral glucose tolerance test tend to be higher when the test is performed in the afternoon than when the test is performed in the morning. Diet High-protein and high-purine diet increase levels of uric acid, urea, and ammonia in blood compared with vegetarians Smoking Long-term smoking increases carboxyhemoglobin, hemoglobin, RBC, WBC and MCV values; WBC level related to number of packs smoked Order the relevant test Know when to order Know how often to order Know how the results will be used If you don t know, consult with the laboratory director Patient Identification JCAHO 2005 Laboratory Services National Patient Safety Goals It is important to identify a patient properly so that a specimen is collected from the correct patient..1% - 1% of specimens are from the wrong patient. Goal: Improve the accuracy of patient identification. Use at least two patient identifiers (neither to be the patient's location) whenever collecting laboratory samples or administering medications or blood products, Use two identifiers to label sample collection containers in the presence of the patient. Proper Patient Identification Procedures Avoiding Specimen Labeling Errors CUMC policy requires two patient identifiers be used in all care settings. Hospital inpatients should be wearing an identification band. Blood should not be drawn from a patient without a identification band. Test forms should be compared to the inpatient's wrist band and to specimen labels or verbally confirmed with an outpatient. Draw and label specimens at the bedside, one patient at a time Affix specimen labels to the specimens immediately after collection Do not draw extra unlabeled tubes The person collecting the specimen should label the specimen Avoid secondary labeling where the specimen is labeled by hand and then printed labels are attached later. 2

3 Specimen Collection Postural Effects Postural Effects Collection Tubes and Additives Effect of Tourniquet Time Collection from IVs and Catheters Volume effects Avoiding Clots Avoiding Hemolysis Change in posture from supine to erect or sitting causes a shift in fluid from the intravascular to the interstitial space of about 12%. An increase of 5% to 15% is seen for most cellular and macromolecular analytes when specimens are collected erect as compared to supine. Conversely, moving from upright to supine can have a dilutional effect owing to an increase in plasma volume The effect of postural change is accentuated in patients with edema. Postural Effects Plastic versus Glass Tubes Albumin levels are higher among healthy outpatients as compared to supine healthy hospitalized subjects Glucose (and other small molecules) move freely between the interstitial space and the circulation and are least affected by posture during blood specimen collection. While the free fraction of a metabolite, drug, hormone, or metal ion is not subject to postural variation, the fraction bound to proteins is affected by posture. Thus, bilirubin bound to albumin and calcium bound to albumin are affected by postural changes. A change from upright to supine can reduce (after 5 minutes) cholesterol level by 10% and triglyceride by 12%. Plastic tubes have replaced glass tubes for most applications Less breakage, cheaper and lower weight Clot activators needed to be added to serum tubes Give clinically equivalent results for almost all analytes Collection Tube Additives Heparin (Anti-Coagulant) Heparin EDTA Citrate NaF + K Oxalate Clot Activator Serum Separator Used to collect whole blood or plasma Binds to anti-thrombin III to inhibit Xa, IXa, and thrombin Nominal concentration of U/mL Heparin binds calcium so ionized calcium must be collected using Calcium Titrated or Electrolyte Balanced heparin 3

4 EDTA (Anti-Coagulant) EDTA Effects K 2 EDTA is used to collect whole blood for hematology studies and plasma for analytes with heparin interference Acts by binding calcium Nominal concentration of 1.5 mg/ml Recent move to K 2 EDTA from K 3 EDTA for hematology to reduce affect on RBC parameters EDTA is hyperosmolar causing cell shrinkage but the low ph of EDTA counterbalances this effect by causing K (and water) to flow into cells. EDTA may cause platelet clumping and platelet satellitism that may be the result of changes in the membrane structure occurring when the calcium ion is removed by EDTA, allowing the binding of preformed antibodies. Sodium citrate tubes are sometimes collected to obtain more accurate platelet counts. Citrate (Anti-Coagulant) NaF + K Oxalate (Anti- Glycolytic and Anti-Coagulant) Citrate is used for collection of coagulation tests Acts by binding calcium Nominal concentration of 3.2% (mol/l) Recent move to 3.2% from 3.8% to get more consistent results for Prothrombin Time, particularly for more sensitive reagents Tubes must be properly filled to within +/- 10% of assigned collection volume NaF + K Oxalate is used to poison glycolytic pathway and to anti-coagulate specimens for glucose testing Glucose decreases by 5 7% per hour in specimens from adults and by up to 24% per hour in specimens from neonates or patients with very high white counts Delay in action leads to approximately 9 mg/dl loss over the first 3 hours after collection Causes a great deal of hemolysis and not suitable for other testing Clot Activator (Shortens Time to Clot) Serum Separator Gel (Separates Serum from Cells) In vitro activation of clotting system to enhance clot formation A silica clot activator is attached to the tube wall with a silicone surfactant Requires mixing for optimal function Time to clot is minutes instead of 1 hour Polymer gel with specific gravity between that of serum (or plasma) and cells Migrates and forms a barrier during centrifugation Separation of cells from serum stops metabolic and hemolytic effects Certain analytes and therapeutic drugs may bind to gel over time 4

5 Collection Tube Additives Order of Draw Heparin EDTA Citrate NaF + K Oxalate Clot Activator Serum Separator Duration of Tourniquet Application (Metabolic and Concentrating Effect) Collection from IVs and Catheters (Dilutional Effect) Application of the tourniquet for >1 minute can result in hemoconcentration, causing an increase in the concentration of large molecules (e.g. serum proteins) that are unable to pass through the capillary wall. Total protein, iron, total lipids and cholesterol increase from 5%-7%, bilirubin increases 8% and AST 9% Prolonged tourniquet application also promotes anaerobic glycolysis resulting in an increase in plasma lactate, a reduction in blood ph, and an increase in blood potassium. Repeated fist clenching during phlebotomy can also cause a 1 2 meq/l increase in potassium. Blood should not be collected proximal to an IV site but preferably from the other arm Heparin may contaminate specimens collected from central lines unless flushed out with blood High glucose and/or low electrolyte values may result from collecting blood an IV or Central Line If questionable results are obtained from a sample collected through a catheter, the results should be verified using a new sample drawn from a different site Collection from IVs and Catheters Example of Contamination with IV Fluid If a syringe is used, a small volume (<=10 ml) syringe is recommended so that clotting in the syringe during phlebotomy is avoided. If samples must be obtained from a catheter, heparin contamination and dilution must be avoided. The line should be flushed with 5 ml of saline and the first 5 ml of blood or six dead space volumes of the catheter discarded. TEST Sodium Potassium Chloride Bicarbonate Urea Creatinine Glucose Protein Albumin RESULTS Yesterday Today REFERENCE INTERVAL UNITS mg/dl mg/dl mg/dl g/dl g/dl 5

6 Avoiding Hemolysis Allow alcohol to dry before collection Use a larger bore needle Mix gently Avoid syringe collection if possible Avoid collection from IVs and catheters Draw slowly when collecting with syringes or from catheters Transport to lab and centrifuge in a timely fashion Syringe Collection (Hemolysis) Visual hemolysis was found in 19% of specimens drawn by syringe, compared to 3% when drawn by an evacuated tube system. The following reduce the incidence of hemolysis: Pumping the plunger 2-3 times prior to collection to loosen the plunger. Using a 3-10 ml syringe Ensuring that the speed of aspiration does not exceed 1mL of air space during collection. Excessive aspiration forces cause hemolysis. Filling the tube by vacuum only. NEVER push down on the plunger; this increases the force of the blood flow, creating a high degree of red blood cell trauma. Using a larger bore needle Not transferring specimen between containers Avoiding Clots Collection Volume Use a sufficient amount of the correct anticoagulant Mix specimen thoroughly after collection Transfer immediately from syringe to tube Do not overfill tubes Overfilled tubes Under filled coagulation tube Under filled hematology tube Under filling occurs because: Tube was removed too quickly Tube slips back from vacutainer needle Air drawn in from butterfly or connector tubing Arterial Blood Gases Specimen Transport Avoid air contamination from a bubble or uncapped specimen Delay in analysis can cause high po 2 to fall or low po 2 to rise Analyze within 30 minutes or place on ice and analyze within 1 hour. Ca2+ binding by heparin can be minimized by using either of the following: (1) A final concentration of sodium or lithium heparinate of 15 IU/ml blood or less (2) Calcium titrated heparin with a final concentration of less than 50 IU/ml blood. Heparin Dilution effect can be avoided by use of dry heparin Roll specimen to mix heparin and reduce clots Specimens should be delivered to the laboratory promptly after collection Specimens should not be placed on ice unless specified by the laboratory Pneumatic tube transport does not affect analytical results 6

7 Iatrogenic Green Plasma due to Isosulfan Blue Dye Learning Objectives Pre-Analytic Issues in Laboratory Medicine What is the impact of posture on laboratory analytes? Be able to describe the common additives to blood specimens and when they are used Be able to describe the common causes of hemolysis and clot formation in the preanalytic setting Discussion 7